Only thing I can think of, then, is to report it to Microsoft as a bug. Unless they tell you that the attendees are deliberately not imported (as a security mechanism, perhaps)—though I don't believe that—I should think it's something they'd be interested in fixing in time for the next release.

Wait, there is one more thing. It's a pain, but I imagine it's possible: Write yourself a VB program that a) imports the CSV into Outlook, b) reads the email addresses from the CSV and c) modifies the Outlook appointment accordingly. It's just a workaround, and it may well be more work than it's worth to you. But at least you can be sure of getting it to work ... eventually.